High-frequency raster with irregular structure of raster elements

ABSTRACT

What is proposed is a high-frequency raster with an irregular structure of raster elements. This raster is a plane-parallel glass plate with transparent roughnesses on its surface characterized by a value Ra equal to 0.20-0.26 mu. 
     The high-frequency raster with an irregular structure of raster elements is preferably prepared by a method in which a glass plate is ground at two sides to obtain a uniform structure of the surface. The ground plate is subjected to etching to obtain transparent microroughnesses with a value Ra equal to 0.20-0.26 mu. 
     The raster proposed by the present invention is characterized by a high resolving power in the order of 25 mm -   1 , transmits a normal interval of optical densities of 1.0 to 1.2 and is capable of reproducing an image with a finely grained discrete structure.

This is a divisional of applicants' co-pending patent application Ser.No. 451,599, filed Mar. 15, 1974, titled "High-Frequency Raster withIrregular Structure of Raster Elements and Method of Manufacturing theSame", now U.S. Pat. No. 3,976,523 dated Aug. 24, 1976.

The present invention relates to polygraphic industry and, moreparticularly, the invention relates to high-frequency rasters with anirregular structure of the raster elements and to a method ofmanufacturing the same.

The rasters are widely used for conversion of a half-tone image into araster image at the stage of a photographic process when producingprinting forms for reproducing an image with an irregular structure ofraster elements.

High-frequency rasters with an irregular structure of their elementshave a number of advantages over the well known and widely used rasterswith a periodic structure of raster elements; in particular, they have ahigher resolving power. An image reproduced by means of such a raster ischaracterized by complete absence of moire. This feature of a rasterwith an irregular structure allows the resolution of an image to beimproved to a considerable extent, thus improving the viasual perceptionof the image reproduction as a whole.

Known in the art is a high-frequency raster with an irregular structureof raster elements comprising a plane-parallel glass plate having atboth its surfaces frosted micro-roughnesses characterized by a value Rain the order of 0.32-0.33 mu.

This raster has a sufficiently high resolving power; however, theinterval of optical densities reproduced by the known raster is low andthis makes it difficult to reproduce different originals.

Known in the prior art is a method of manufacturing a high-frequencyraster with an irregular structure of raster elements, which is effectedby means of bilateral etching of a glass plate by vapours ofhydrofluoric acid. The plate is treated in vapours of hydrofluoric aciduntil there is obtained on its surface a microstructure with a value ofmicroroughnesses characterized by Ra equal to 0.32-0.33 mu. This methodof manufacturing a raster has significant disadvantages. The nonuniformswirling evaporation of hydrofluoric acid in the process of etching theglass results in that it is difficult to obtain a surface with a similarmean-statistical value Ra of microroughnesses over the whole area of theplate being processed. This results in nonuniformity of themean-statistical characteristic of the structure of the raster image anddeteriorates its quality.

The microstructure of the glass surface treated with hydrofluoric acidis matted. Such a character of microroughness on the raster surfaceresults in a low (in the order of 0.7) interval of optical densitiestransmitted by the raster.

An object of the present invention is to provide a high-frequency rasterwith an irregular structure which is capable of creating a discreteimage on a recording photographic layer.

Another object of the invention is to provide a high-frequency rasterwith an irregular structure which is characterized by a high resolvingpower.

Still another object of the invention is to provide a high-frequencyraster with an irregular structure which transmits a normal interval ofoptical densities.

These and other objects are attained by providing a high-frequencyraster with an irregular structure of raster elements which comprises aglass plate having microroughnesses on both its surfaces; according tothe invention, the value of the microroughnesses characterized by Ra isequal to 0.20-0.26 mu.

The proposed raster has a high resolving power in the order of 25 mm⁻ ¹,is capable of producing a discrete image on a recording photographiclayer, and can transmit an interval of optical densities of 1.0-1.2.

It is expedient to make a high-frequency raster with an irregularstructure of raster elements by a method in which a glass plate isetching both sides with hydrofluoric acid and, according to the presentinvention, prior to the etching, the plate is ground at two sides toobtain a uniform structure of its surfaces; the etching is effectedrepeatedly until microroughnesses are obtained which are characterizedby a value Ra equal to 0.20-0.26 mu.

Using the proposed method, it is possible to obtain a transparent rasterproducing a high-frequency raster image on a recording photographiclayer, having a resolving power in the order of 26 mm⁻ ¹ andtransmitting an interval of optical densities of 1.0-1.2.

According to the present invention, the plate is preferably etched atleast three times.

The etching of the glass plate at least three times provides foruniformly distributed microroughnesses of a specified value on anoptically cleared surface.

One embodiment of the present invention involves etching of the plate ina solution containing 4 parts by volume of hydrofluoric acid and 1 partby volume of sulphuric acid.

Another embodiment of the invention consists in that the plate issubjected to etching in a solution comprising 3 parts by volume ofhydrofluoric acid and 1 part by volume of phosphoric acid.

Still another embodiment of the invention consists in that the plate issubjected to etching in a solution containing 4 parts by volume ofhydrofluoric acid, 1 part by volume of sulphuric acid and 1 part byvolume of water.

When using the above-mentioned solutions, it is possible to carry outuniform etching of the whole surface of the plate and to obtain a uniquecharacteristic of the microroughnesses.

Given below is a detailed description of the present invention fromwhich one can see further advantages of the proposed high-frequencyraster with an irregular structure of raster elements and a method ofmanufacturing such rasters.

The high-frequency raster with an irregular structure of raster elementsis made of a plane-parallel glass plate, both surfaces of which havemicroroughnesses whose Ra is equal to 0.20-0.26 mu.

It is well known that such microroughnesses are characterized by a valueRa which gives an impression of a mean square value of deviation of themicroroughnesses from the average line of their profile.

The raster proposed in the present invention has these microroughnesseswith the same mean-statistical value Ra over the whole surface of theglass plate, which results in uniformity of the mean-statisticalcharacteristic of the raster image.

The high resolving power of the raster is equal to 25 mm⁻ ¹, the normalinterval of optical densities transmitted by the raster and thecapability of creating a discrete structure of an image on a recordingphotographic layer are attained due to the fact that the raster has onits surface transparent microroughnesses whose value is equal to0.20-0.26 mu.

According to the present invention, the high-frequency raster with anirregular structure of raster elements is prepared as follows.

A plane-parallel glass plate having a thickness of 1 to 10 mm is groundat both sides to obtain a surface with the same mean-statistical valueRa of microroughnesses and to produce microroughnesses with parametersthat determine the frequency and size of the future raster elements on araster image. However, such a treatment of the surface of the glassplate results in a matted microstructure so that the application of araster with such a microstructure reslts in dispersion of light andreduction of the interval of optical densities of a half-tone image.

According to the invention, the microroughnesses with an opticallycleared, transparent surface are produced by subjecting the ground plateto chemical etching. It is expedient to use an etching solutioncontaining 4 parts by volume of hydrofluoric acid, 1 part by volume ofsulphuric acid, or a an etching solution containing 3 parts by volume ofhydrofluoric acid and 1 part by volume of phosphoric acid, or an etchingsolution containing 4 parts by volume of hydrofluoric acid, 1 part byvolume of sulphuric acid and 1 part by volume of water. The aboveetching solutions provide for a uniform and gradual etching of thesurface being treated.

A finely grained structure with a high uniformity over the whole surfaceof the raster being made is obtained by means of multiple etching. Theplate is preferably be etched at least three times.

Single etching of the plate results in formation of a coarse structuresof the surface and, therefore, reduces the resolving power of the rasterand deteriorates visual perception of the raster image.

The etching is followed by washing in water. This treatment is carriedout until transparent microroughnesses are obtaines whose value Ra isequal to 0.20-0.26 mu; these roughnesses are capable of producing adiscrete image on a recording layer.

Such a combined treatment makes it possible to obtain transparenthigh-frequency rasters with an irregular structure of raster elementshaving a high resolving power in the order of 25 mm⁻ ¹, transmitting anormal interval of optical densities of 1.0-1.2, and reproducing imageswith a finely grained discrete structure with irregular raster elements(variable in area, quantity and shape).

The proposed rasters can be employed in contact-type copying devices.The high-frequency transparent rasters can also be used for reproductionof artistic originals with a great amount of fine details of an image,since they make it possible to considerably increase the accuracy of thetransmission due to a high resolving power and the absence of moire.

EXAMPLE 1

A plane-parallel glass plate is ground at both sides by an abrasivepowder, each side being ground during 40 minutes; then the ground plateis carefully washed and subjected to chemical treatment in a solution ofthe following composition 4 parts by volume of hydrofluoric acid, 1 partby volume of sulphuric acid.

The etching is carried out in 5 stages; in the intervals between thesestages the plate is washed with water. After this treatment, transparentmicroroughnesses are obtained on the surface of the glass plate, whosevalue Ra is equal to 0.22 mu.

Due to the above-said combined treatment there is obtained a transparentraster with an irregular structure of raster elements reproducing aninterval of optical densities of an image of 1.1-1.2 and with aresolving power of 25 mm⁻ ¹.

EXAMPLE 2

A plane-parallel plate is ground at two sides, each side being groundduring 40 minutes. After careful washing of the plate it is subjected tochemical etching in a solution having the following composition: 3 partsby volume of hydrofluoric acid and 1 part by volume of phosphoric acid.The treatment is carried out in 4 stages; the etching stages arealternated with washing of the plate in water. On the surface of theglass plate there are produced transparent microroughnesses whose valueRa is equal to 0.24 mu.

Thus, the obtained high-frequency raster with an irregular structure ofraster elements transmits an interval of optical densities of 1.0-1.1and has a resolving power of 25 mm⁻ ¹.

EXAMPLE 3

A plane-parallel glass plate is ground at two sides with an abrasivecompound, each side being ground during 40-45 minutes. After washing theground plate with water, it is treated with a solution of the followingcomposition: 4 parts by volume of hydrofluoric acid, 1 part by volume ofsulphuric acid and 1 part by volume of water. The etching is effected 5times with intermediate washing of the plate with water. As a result,there is obtained a high-frequency raster with an irregular structure ofraster elements with transparent micro-roughnesses on the raster surfacehaving a value Ra equal to 0.24 mu.

The obtained raster transmits an interval of optical densities of 1.1and has a resolving power of 25 mm⁻ ¹.

What is claimed is:
 1. An optical high-frequency raster with anirregular structure of raster elements, the raster comprising asubstantially plane-parallel glass plate mechanically ground on bothsides to obtain a uniform surface structure and having transparentmicroroughnesses on both surfaces, the microroughnesses having a valueRa equal to 0.2 to 0.26 mμ, washed with water and subjected to multipleetching, at least three times, to obtain the transparentmicroroughnesses, whereby the resulting raster has a high resolvingpower of at least 25 mm⁻ ¹, and is capable of transmitting an intervalof optical densities between approximately 1.0 and 1.2.